The invention relates to a process for the separation of traces of gaseous contaminants, or pollutants, based on halogens and/or sulfur and/or their compounds from waste gases by chemical absorption.
Waste gases resulting from the operation of plants in the chemical industry, such as cellulose plants, sewage-treatment plants, nuclear power plants and similar plants, usually contain gaseous contaminants which, depending on their chemical composition, may be malodorous, unpleasant, noxious or toxic and thus injurious to the environment. Therefore they must be removed from the waste gases.
The gaseous contaminants produced in the industrial plants named are mainly sulfur- or halogen-containing compounds and, more particularly, H.sub.2 S, mercaptans, HCl, HBr, HI, certain sulfur and halogen organic compounds, chlorine, bromine and iodine.
One of the most widely used processes for the removal of contaminants of the types named from waste gases has been absorption.
Absorption processes suitable for the purification of waste gases are characterized in that they use wash liquids.
To deodorize waste gases with sulfur-containing contaminants, even minute traces of the latter must be removed since the threshold of odor is very low, particularly in the case of mercaptans. For ethyl mercaptan, for example, it is 1.6.times.10.sup.-5 -2.times.10.sup.-3 ppm.
Similarly, even minute traces of iodine contained in the waste gases from nuclear power plants must be removed because of their radioactivity.
In the case of oxidizable contaminants, a method commonly employed to improve the efficiency of such absorption processes, and especially to remove traces, is the oxidative gas wash. (See Proposed VDI Guidelines, VDI 2443, 1977, Waste-Gas Purification Through Oxidizing Gas Wash.)
This method can be used both with readily volatile oxidizing agents such as chlorine, chlorine dioxide and ozone and with nonvolatile oxidants such as H.sub.2 O.sub.2, sodium chlorite and potassium permanganate.
The use of readily volatile oxidizing agents does result in rapid oxidation of the contaminants dissolved in the wash liquid; however, this method has the drawback that when the amount of contaminants in the waste gas fluctuates, stoichiometric metering of these volatile substances is extremely difficult, excess oxidant thus being emitted, which instead of solving the waste-gas problem gives rise to an additional health hazard.
While the use of nonvolatile oxidizing agents largely eliminates the above problem, this method in turn has the disadvantage that the reactivity is often too low to permit rapid oxidation.
Another widely used variant of the absorption process is the alkaline wash, which can be employed also with contaminants such as chlorine, bromine and iodine. The drawback of this process is that CO.sub.2 present in the effluent air is also washed out, which is undesirable.
A combination oxidation/alkaline wash process is described by H. Kurmeier in Gesundheits-Ing. 92 (1971), 6, 169-173.